1. Field of the Invention
The present invention relates to a device for measuring the distance between a head and a headrest, headrest position adjusting device using said device, method for measuring the distance between a head and a headrest, and headrest-position adjusting method using said method.
2. Description of the Related Art
Conventionally, a headrest device is known as a device for control or adjust the position of a headrest arranged on a seat of a vehicle such as an automobile, etc. (for example, see JP A-2006-181233 (pp. 2-9, and FIGS. 1-13)). This headrest device is provided with a receiving portion for receiving a head of a vehicle occupant sitting on the seat of the vehicle, and a drive unit that enables the receiving portion to move near to the position of the head.
The drive unit is provided with a sensor arranged in the receiving portion and sensing a relative position of the head, a connecting mechanism constructed to move the receiving portion back and forth, and up and down relative to the seat of the vehicle, and controlling means for driving and controlling the connecting mechanism. The connecting means move the receiving portion relatively to a first relatively-adjacent position in which a sensor shows excellent sensing sensitivity, and then to a second relatively-adjacent position based on the sensing signal from the sensor.
A vehicle headrest device is known (for example, see JP 4018112 (pp. 1-15, and FIGS. 1-12). When this vehicle headrest device moves the front portion of the headrest in a direction to be fully opened, ECU of the device judges based on the detection result of a capacitance sensor that the front portion of the headrest comes close to the head of a vehicle occupant, and stops the front portion of the headrest.
Concretely, ECU judges based on change in an absolute capacitance relative to the standard capacitance of a capacitance sensor that the front portion of the headrest comes close to the head of a vehicle occupant. Alternatively, the headrest device is constructed such that ECU judges based on change in capacitance value of a capacitance sensor that the front portion of the headrest comes close to the head of a vehicle occupant.
However, the headrest device disclosed in JP A-2006-181233 described above measures the distance between the head and the receiving portion based on the absolute value of the capacitance which is easily affected by outer noise due to change in environment such as temperature or humidity sensed by one sensor. Therefore, there is a problem of difficulty in measuring the precise distance between the head and the headrest.
Further, in the vehicle headrest device disclosed in JP 4018112 described above, the distance between the headrest and the head of the vehicle occupant is measured based on change in capacitance when the front portion of the headrest is minutely moved back and forth. By doing so, the distance can be measured without being affected by outer noise. However, in this case, it is necessary to move constantly the front portion of the headrest. For that reason, the headrest moves wastefully, which makes the vehicle occupant feel uncomfortable. Further, there is a problem that makes it difficult to measure accurately the distance when the position of the headrest moves during measurement.